The parathyroid glands regulate calcium homeostasis. The major target organs for parathyroid hormone (PTH) are bone and the kidneys. PTH increases bone resorption to mobilize calcium into the circulation. In the kidney, PTH enhances calcium reabsorption and increases phosphate excretion. PTH also stimulates renal production of 1α,25-dihydroxyvitamin D [1,25(OH)2D], which in turn enhances intestinal absorption of calcium. Thus, the physiological effects of PTH are to increase the concentration of calcium in the circulation. Negative feedback from calcium and 1,25(OH)2D modulate parathyroid function. These effects are mediated via the calcium-sensing receptor (CaSR) expressed on the parathyroid cell surface. Similarly, 1,25(OH)2D, acts through vitamin D receptor (VDR) and suppresses PTH synthesis and secretion . Thus, PTH secretion is tightly coupled to the parathyroid cell's ambient calcium level.